If the sympton appears while not under load, that is when you hit the gas in neutral, it is quite likely the acc. pump. If you had an air leak in the manifold, it wouldn't idle very well and would ping under load. This I have found from experience1

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!